This invention relates to a process and an apparatus for selective deposition of a metal thin film such as a tungsten thin film on a specified area of a substrate, particularly with high selectivity and high rate.
With higher integration of LSI, there have become finer individual elements and wiring, or diameters of contact-holes or through-holes formed on insulating films for contacting wiring with each other. On the other hand, since the thickness of insulating films cannot be reduced, a ratio of the depth to the diameters of these holes (aspect ratio) becomes larger, which results in making it remarkably difficult to fill up the holes with a conductive metal. For example, according to a sputtering method of aluminum which is the most general method for forming a metal film, the diameter of 2 .mu.m and the aspect ratio of about 1 is the upper limit of filling up. In order to fill up holes having smaller diameter and larger aspect ratio, other methods should be applied. One of these methods is a selective chemical vapor deposition (CVD) method of a metal, typically tungsten (W), which method has been studied and many reports have been published.
According to a selective CVD method of W, tungsten hexafluoride (WF.sub.6) and hydrogen (H.sub.2) are used as starting material gases, which are introduced with a predetermined pressure and in a certain amount into a reactor wherein a heated substrate is installed. On the portions exposing silicon (Si) underlayer of the substrate, a W film is deposited by the following Si reduction reaction: EQU WF.sub.6 +3/2Si.fwdarw.W+3/2SiF.sub.4 .uparw. (1).
By the Si reduction reaction, the W film can be grown up to several hundreds angstroms in thickness. Further, aluminum (Al) can also form a W film by directly reacting with WF.sub.6 like Si. Since a catalytic action as to the adsorption and dissociation of H.sub.2 takes place on the conductor film such as W film formed by the formula (1), the following reduction reactions proceed by H atom to continuously grow the film: EQU H.sub.2 .fwdarw.2H (on catalyst surface) (2) EQU WF.sub.6 +6H.fwdarw.W+6HF (3).
Further, since adsorption and dissociation of H.sub.2 of the above formula (2) take place even on a conductor film of MoSi.sub.2, WSi.sub.2, PtSi, etc., a W film is deposited and grows. The above-mentioned reaction proceeds at a substrate temperature of about 200.degree. C. or higher.
On the other hand, the Si reduction reaction of (1) does not take place on an insulating film of SiO.sub.2, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3 or the like. Further, since the catalytic action as to the adsorption and dissociation of H.sub.2 at about 700.degree. C. or less on such an insulating film does not take place, the dissociation of H.sub.2 by the formula (2) does not take place to proceed the reduction reaction by the H atom, so that no metal thin film is formed. Therefore, according to the selective CVD method using a metal halide such as WF.sub.6, or the like and H.sub.2 as starting material gases, a metal thin film is selectively deposited on a underlayer of Si or a conductor metal, so that it is possible in principle to fill up holes however fine and deep these holes may be so long as the starting material gases are supplied into the holes.
But, according to a prior art selective CVD method for a metal thin layer, there takes place undesirably a phenomenon that a metal is deposited even on an insulating film of SiO.sub.2, or the like.
As an apparatus for the selective CVD method for forming a metal thin film typified by a W thin film, there has been used a low pressure CVD apparatus excellent generally in a film thickness distribution and step coverage properties. In such a case, considering the selective formation of a metal thin film, it was necessary to make some device so as not to form a metal thin film on a reactor wall or the like other than the substrate. As the low pressure CVD apparatus, there are a hot-wall type and a cold-wall type.
The hot-wall type CVD apparatus is characterized by heating the whole reactor with a heater, and has an advantage in that infrared light from the heater transmits the reactor, the interior of which is heated uniformly. Further, in the case of forming a metal thin layer by using a metal halide gas such as WF.sub.6, or the like and H.sub.2, the selective film formation becomes possible by using quartz which suppress the formation of the metal thin layer as the reactor. But there is a problem in that when there are contaminations which become nuclei for film formation on the inner wall of the reactor even in trace amounts, the film formation area of metal thin film is enlarged around the nuclei as their centers and the metal thin film is finally formed on portions of the substrate not desired to be formed.
On the other hand, the cold-wall type CVD apparatus is characterized by cooling the whole reactor with water, while heating a substrate with an infrared lamp from a back side of the substrate, on the desired portion of front side of which is formed a metal thin film, together with substrate supporting units. According to a process for using such a cold-wall type CVD apparatus, there are advantages in that since heated portions other than the desired substrate surface on which a metal thin film is to be formed are not exposed to the starting material gases, the reaction between the reactor wall and the starting material gases does not take place and the film-forming rate is stable. Further, since the substrate is heated together with the substrate supporting units, there is an advantage in that the substrate surface temperature can be maintained uniformly. But there is a problem in that since the substrate supporting units are also heated, a metal thin film is also formed on the surface of substrate supporting units and a metal thin film formation area is enlarged therefrom so as to form a metal thin film on undesired portions of the substrate.
As mentioned above, according to the prior art processes, it was difficult to form thin films selectively on only the desired portions while maintaining good selectivity with good reproducibility without fail. In order to improve the selectivity in the prior art processes, it is possible to employ as general consideration a low temperature for the treatment (lower than 350.degree. C.), a short deposition time, careful cleaning of a substrate surface, a small surface for deposition, etc. But there may bring about lowering in throughput and a limitation to the applications for processes. This is contrary to the desire to carry out selective film formation for obtaining any desired film thickness with a high film-forming rate while maintaining good selectivity. Selective CVD of W is disclosed, for example, in J. Electrochemical Society, vol. 131 (1984), pp. 1427-1433; Proc. 2nd. Int. IEEE VLSI Multilevel Interconnection Conf. vol. 25 (1985), pp. 343, etc. Further, apparatus for selective CVD of W is disclosed, for example, in U.S. Pat. No. 4,547,404, etc.
Further, the formation of a metal thin film on an insulating film of SiO.sub.2 or the like using a metal halide gas and H.sub.2 as the starting material gases is difficult as mentioned above in principle. But as disclosed in Extended Abstracts of the Meeting of 170th Electrochem. Soc. vol. 86-2, pp. 500 (1986, Oct.), when H atoms are produced in a gas phase by using H.sub.2 plasma, etc., a metal thin film can easily be formed even on an insulating film of SiO.sub.2, or the like. That is, since there is no catalytic action of adsorption and dissociation of H.sub.2 on the insulating film of SiO.sub.2 or the like, the dissociation reaction of the above-mentioned formula (2) does not take place. But by adhering H atoms produced in the gas phase to the insulating film of SiO.sub.2 or the like, the reaction of formula (3) proceeds even on the surface of SiO.sub.2 or the like to form a metal thin film of W. In other words, when H atoms are present in the gas phase from some causes and adhere to a surface of SiO.sub.2 portions on which the formation of W or the like film is not desired, a metal is deposited thereon. But according to prior art selective CVD for forming metal thin films and apparatus used therefor, the prevention of the adhering of H atoms to surface portions on which the formation of the film is not desirable was not considered.
As mentioned above, according to the prior art technique, the prevention of degradation of selectivity, that is, the prevention of adhering of H atoms to an insulating film, this being a cause for depositing the metal on SiO.sub.2 or the like insulating film on which the deposition of the metal is not desirable, was not considered and the metal thin film was not formed with good reproducibility, good selectivity and high rate.